The present invention generally relates to an image processing apparatus, and more particularly to an image processing apparatus in which multilevel image data obtained from a composite image through raster scanning is subjected to an MTF (modulation transfer function) correction process and a bi-level rendition process for improving the picture quality of bi-level image portions of the multilevel image data while it is subjected to a gamma correction process and a pseudo halftone rendition process for improving the picture quality of halftone image portions of the multilevel image data.
In image processing apparatuses such as facsimile machines, a multilevel image data is read from an original image via raster scanning, the multilevel image data is converted into a bi-level image data through a bi-level rendition process, and the resultant bi-level image data is transmitted, received or temporarily stored. Since a certain amount of the original image information is lost when the bi-level rendition process is performed, the picture quality may be seriously degraded when an output image is output by the image processing apparatus from such a bi-level image received from an external facsimile unit or temporarily stored within the image processing apparatus. For example, a faded portion appears in the output image due to the bi-level rendition process having been performed. For this reason, there is a problem in that the picture quality of the hi-level image becomes worse than that of the original image.
In order to prevent the picture quality of the output image from being degraded due to the bi-level rendition process through which the input multilevel image data is converted into the output bi-level image data, several improving attempts have been made. For example, an MTF correction process (which will be described below) is performed with respect to the bi-level image in order to reduce the faded portion in the output image. A pseudo halftone rendition process to which an error diffusion method is applied (which process will be described later) is performed with respect to the halftone image in order to improve the picture quality of the output image. A gamma correction process is performed with respect to multilevel image data read from a halftone image such as a photograph, in order to reduce differences between the input image gradation characteristics of the image input part and the output image gradation characteristics of the image output part. With respect to the resulting multilevel image data due to the gamma correction process, a pseudo halftone rendition process or the like is performed.
In a case in which the multilevel image data is obtained from a composite image in which a bi-level image and a halftone image coexist on one page, the MTF correction process and the simple bi-level rendition process are performed with respect to a bi-level image portion of the multilevel image data while the gamma correction process and the pseudo halftone rendition process are performed with respect to a halftone image portion thereof in order to prevent the output image from having a poor picture quality.
FIG. 1 shows a conventional image processing apparatus in which the above mentioned correction processes are performed. In FIG. 1, the multilevel image data obtained from an original image through the raster scanning is supplied per one scanning line to both a line buffer 1001 and a gamma corrector 1002. In the line buffer 1001, the multilevel image data corresponding to a number of scanning lines is temporarily stored, and the stored image data is supplied per one scanning line to an MTF corrector 1003. The MTF corrector 1003 performs the MTF correction process with respect to the image data received, and a bi-level rendition part 1004 performs the bi-level rendition process with respect to the resulting image data so that the multilevel image data is converted into the bi-level image data.
The gamma corrector 1002 performs the gamma correction process with respect to the multilevel image data received, and the resulting image data is supplied to a line buffer 1005. In the line buffer 1005, the multilevel image data corresponding to a number of scanning lines is temporarily stored, and the stored image data is supplied per one scanning line to a pseudo halftone rendition part 1006. The pseudo halftone rendition part 1006 performs the pseudo halftone rendition process so that the multilevel image data received is converted into the bi-level image data. An image synthesizer 1007 selectively receives the bi-level image data from the bi-level rendition part 1004 which data corresponds to a bi-level image portion of the original image, and selectively receives the bi-level image data from the pseudo halftone rendition part 1006 which data corresponds to a halftone image portion of the original image. Then, the image synthesizer 1007 produces an output image with respect to one page from these bi-level image data being received.
However, in the case of the above described apparatus, there is a problem in that the picture quality of the bi-level image cannot be sufficiently improved after the correction processes are performed therefor. Also, there is a problem in that the manufacture of the above described apparatus is expensive because it requires two line buffers for performing different correction processes with respect to each of the bi-level image portion and the halftone image portion contained in one page of the composite image.